Lamp assemblies including light emitting diodes (LEDs) may be configured to exhibit increased energy efficiency and longer life compared to conventional incandescent lamps. LED lamps have therefore become desirable as a replacement for conventional incandescent lamps. One challenge associated with the design of LED lamps relates to management of the heat generated by the LEDs. To conduct the heat away from the LEDs, LED lamp assemblies typically incorporate a heat sink that is thermally coupled to the LEDS. The effectiveness of the thermal coupling of the LEDs to the heat sink affects the lumen efficiency and longevity of the LEDs in the assembly. Examples of LED lamp assemblies incorporating a heat sink configuration may be found, for example, U.S. Patent Pub. No. 2011/0111536 (Brunner et al.); U.S. Pat. No. 7,922,364 (Tessnow et al.); U.S. Patent Pub. No. 2010/0207505 (Tessnow); U.S. Pat. No. 7,806,562 (Behr et al.); U.S. Patent Pub. No. 2009/0034283 (Albright et al.); U.S. Pat. No. 7,357,534 (Snyder); U.S. Pat. No. 7,261,452 (Coushaine et al.); U.S. Patent Pub. No. 2007/0070645 (Coushaine et al.); U.S. Pat. No. 7,110,656 (Coushaine et al.); U.S. Pat. No. 6,991,355 (Coushaine et al.); and U.S. Patent Pub. No. 2005/0243559 (Coushaine et al.).
FIG. 1 is an exploded view of a known LED lamp assembly 100. The illustrated assembly includes a metallic heat sink 102, a connector 104, a thermal pad 106, a printed circuit board (PCB) 108 with LEDs 110 and associated electronics thereon, a housing cover 112 including an integrally molded optic portion 114, and a reflector seal 116. In general, the PCB 108 is provided between the housing cover 112 and the heat sink 102 with the LEDs 110 on the PCB 108 positioned for emitting light through the optic portion 114. Electrical energy for driving the LEDs 110 is coupled through the connector 104 to the PCB 108.
The metallic heat sink 102 is a die-cast part formed from an aluminum alloy. The heat sink includes a body portion 120 having an upper outwardly facing surface 122 and a lower outwardly facing surface 124. The lower outwardly facing 124 surface includes a plurality of pins 126 extending therefrom in a direction away from the lower outwardly facing surface 124 for assisting in dissipating heat conducted to the heat sink 102 from the PCB 108. An opening 128 extends from the upper outwardly facing surface 122 to the lower outwardly facing surface 124. The opening 128 is sized and dimensioned for receiving the connector 104.
The connector 104 includes a hollow body portion 130 and an end portion 132 at the end of the body portion 130. The end portion 132 extends outwardly relative to the body portion 130. A plurality of conductive compliant connector pins 134 extends through the end portion 132 and into a cavity defined by the hollow body portion 130 for electrical connection to a mating connector associated with a power source. The pins 134 extend outwardly from the end portion 132 for electrical connection with a mating connector on the PCB 108. The pins 134 are generally compliant to accommodate compression of the pins 134 into the PCB 108 while maintaining reliable electrical connections.
The connector 104 is received in the opening 128 in the heat sink 102 so that the body portion 130 extends outwardly away from the lower outwardly facing surface 124 of the heat sink body portion 120. The upper outwardly facing surface 122 of the body portion 120 defines a connector channel 136 for receiving an associated rib extending downwardly from a bottom surface 132 of the end portion of the connector 104. The connector 104 is coupled and sealed to the heat sink 102 by inserting a sealant 138 in the channel 136 and registering the rib on the bottom of the end portion 132 of the connector 104 with the channel 136. The sealant 138 may be a sealant that is lubricious prior to curing to facilitate positioning of the connector 104 relative to the heat sink 102, but cures to a relatively hardened state after the connector 104 is registered with the heat sink 102. One material useful as the sealant is the LOCTITE 5910 brand silicone sealant material, which is commercially available from Henkel AG & Co. KGaA of Düsseldorf, Germany.
The heat sink 102 also defines a continuous radial outermost peripheral portion 140 extending outwardly away from the upper outwardly facing surface 122 at the radially outermost periphery of the upper outwardly facing surface 122. A continuous or substantially continuous channel 142 is defined between the outermost peripheral portion 140 and an inner rib 144 extending outwardly away from the upper outwardly facing surface 122 at a position radially inward from the outermost peripheral portion 140. The space radially inward from the outermost peripheral portion 140 between the outermost peripheral portion 140 and upper outwardly facing surface 122 defines a circuit board receiving region 143 surrounded by the channel 142.
The PCB 108 is received within the circuit board receiving region 143. The thermal pad 106 is disposed on the bottom surface 146 of the PCB 108, either as a separate component, or affixed to the bottom surface 146 of the PCB. The thermal pad 106 is constructed from a thermally conductive material to facilitate transfer of heat from the bottom surface 146 of the PCB 108 to the upper outwardly facing surface 122 of the heat sink 102. In one configuration, the thermal pad 106 is provided by printing, e.g. screen printing, a thermally conductive material on the bottom surface 146 of the PCB 108 whereby the material is printed in a liquid state and then cures to non-liquid state. One material useful as a material for the thermal pad 106 is the TC-3500-SP-S brand printable thermal pad material, which is commercially available from Dow Corning Corporation of Midland, Mich., USA.
To facilitate positioning of the PCB 108 and thermal pad 106 in the circuit board receiving region 143, the heat sink defines a plurality of mechanical registration features 148 extending upwardly from the upper outwardly facing surface 122 thereof. The mechanical registration features 148 are sized and shaped to register with corresponding registration openings 150 defined in the PCB 108 and the thermal pad 106. The registration openings 150 in the PCB 108 and the thermal pad 106 are aligned with the mechanical registration features 148 and the PCB 108 is placed into the circuit board receiving region 143 with the mechanical registration features 148 extending into or through the registration openings 150. With the PCB 150 in this position, the bottom surface 152 of the thermal pad 106 is in direct physical contact with the upper outwardly facing surface 122 of the heat sink 102 and the peripheral edge surface 154 of the PCB 108 is in opposed facing relationship to the inner surface 156 of the inner rib 144 of the heat sink 102.
With reference also to FIG. 2, the housing cover 112 is molded from a plastic material. The housing cover 112 includes a longitudinally extending median region 158 having lower surface 160 facing the PCB 108 and an upper surface 162 facing away from the PCB 108, and defines a continuous peripheral rib 164 depending from the lower surface 160 at the radially outermost periphery of the median region 158. The peripheral rib 164 is sized and shaped to register with the channel 142 in the heat sink 102.
The integrally molded optic 114 of the housing cover 112 is positioned centrally in the median region 158. The optic 114 includes an upper portion 166 extending upwardly above the upper surface 162 of the median region 158 and a lower portion 168 extending downwardly away from the lower surface 160 of the median region 158 and toward the PCB 108. The reflector seal 116 is an annular rubber gasket element disposed around the upper portion 166 of the optic and against the upper surface 162 of the median region 158 for sealing the assembly 100 to a mounting position, e.g. in a vehicle head lamp.
The optic 114 defines a passage 170 extending from a top surface 172 of the optic 114 to a bottom surface 174 of the optic 114. A plurality of reflectors 176 is disposed in the passage 170. When assembled, the lower portion 168 of the optic is disposed at a predetermined nominal distance from the LEDs 110 with the optical axis A of the optic 114 transverse to the median region 158 and aligned with the LEDs 110. The lower portion 168 of the optic 114 includes respective openings 178 aligned with each of the LEDs 110 whereby light emitted by the LEDs 110 passes into the openings 178, through the lower portion 168 of the optic 114, and out of the passage 170 through the upper portion 166 of the optic 114 with at least some of the light being reflected out through the upper portion 166 by the reflectors 176.
A plurality of registration pads 180 are defined on the lower surface 160 of the median portion 158 adjacent the optic 114. The registration pads 180 extend downwardly toward the PCB 108. In an assembled condition, the registration pads 180 limit a spaced relationship between the optic 114 and the LEDs 110.
A plurality of mechanical registration projections 182 are defined on the lower surface 160 of the median portion 158 adjacent each side of the bottom portion 168 of the optic 114 and closer to the bottom portion 168 of the optic than to the peripheral rib 164 of the housing cover 112. Each of the mechanical registration projections 182 is configured as a hollow receptacle that extends downwardly toward the PCB 108 with an opening 184 that is sized and shaped to register with the mechanical registration features 148 of the heat sink 102. The mechanical registration features 148 on the heat sink 102 thus register with the registration openings 150 in the thermal pad 106 and the PCB 108 and the mechanical registration projections 182 on the housing cover 112 to provide a desired alignment between the housing cover 112, the PCB 108 and the heat sink 102.
The housing cover 112 illustrated in FIGS. 1 and 2 was injection molded in an edge-gated fashion, i.e. plastic was injected into a mold at the edge of the housing cover, at a location along a line (the Y-Y cross-sectional line in FIG. 4) perpendicular to the longitudinal axis of the reflectors 176. The housing cover 112 was molded from a polybutylene terephthalate (PBT) material such as the CRASTIN CE2055 NC010 brand material, which is commercially available from the E. I. DuPont de Nemours & Co., Inc., of Wilmington, Del., USA. The molded housing cover 112 was without warp. The term “warp” as used herein refers to a departure from a planar surface that is dished inward toward the PCB 108 and is measured according to the method described below in connection with FIG. 3 and FIG. 4.
Warp as described herein is determined by making a plurality of measurements along the upper surface 162 of the median region 158 in a cross-section of the housing cover 112. The measurements may be made in any cross-section extending across the upper surface 168 of the median region 158. For example, measurements may be made in the cross-section extending through the longitudinal axis of the reflectors 176, i.e. the X-X cross-section in FIG. 4, or in the cross-section that is 90 degrees to the longitudinal axis of the reflectors 176, i.e. the Y-Y cross-section in FIG. 4.
FIG. 3 diagrammatically illustrates a side view of the housing cover 112 of FIG. 2 with measurement lines L1 and L2 in the Y-Y cross-section and with warp in the housing cover 112 exaggerated as to scale of ease of explanation. Although measurements are described herein as being made in the Y-Y cross-section it is to be understood that measurements in other cross-sections are made in the same manner.
As shown, a measurement M1 is made at a first side 190 of the upper portion 166 of the optic 114 from a tangency T1 to the median region 158 where the upper surface 162 of the median region 158 intersects the upper portion 166 of the optic to the line L1, which is parallel to the tangency T1 and intersects the outermost peripheral edge 192 of the top surface 162 of the median region 158 on the first side 190 of the upper portion 166 of the optic 114. A second measurement M2 is made at a second side 194 of the upper portion 166 of the optic 114 from a tangency T2 to the median region 158 where the upper surface 162 of the median region 158 intersects the upper portion 166 of the optic 114 to the line L2, which is parallel to the tangency T2 and intersects the outermost peripheral edge 192 of the top surface 162 of the median region 158 on the second side 194 of the upper portion 166 of the optic 114. Warp is calculated for the cross-section in which measurements are made by taking an average of the measurements M1 and M2. The warp in housing cover 112 is the warp calculated in the cross-section that gives the maximum warp value. A housing cover 112 is described herein as having “zero” warp or “without” warp, when warp calculated by the method described above is less than about 0.1 mm.
The housing cover 112 is coupled and sealed to the heat sink 102 by inserting the sealant 138 in the channel 142 and registering the peripheral rib 164 of the housing cover 114 with the channel 142 in the heat sink 102. As discussed above, the sealant 138 may be lubricious prior to curing to facilitate mating of the housing cover 112 with the heat sink 102, but may cure to a relatively hardened state after the housing cover is mated with the heat sink 102. The housing cover 112 may be further secured to the heat sink 102 by fasteners, such as screws, a snap-fit of corresponding elements on the heat sink 102 and the housing cover 112 and/or by staking.
FIGS. 4 and 5 illustrate the lamp assembly 100 in an assembled condition. When assembled, the outermost peripheral portion 140 of the heat sink 102 forms a generally square corner with the top surface 162 of the median region 158 of the housing cover 112 without imparting compressive force on the peripheral rib 164 of the housing cover 112. In addition, warp in the housing cover 112 is generally dished inward toward the heat sink 102.
One difficulty associated with the LED lamp assembly 100 relates to achieving a desired tolerance between the bottom surface 174 of the optic 114 and the LEDs 110. In one embodiment, for example, the desired height between the bottom surface 174 of the optic and the top of the LEDs 110 is 0.8 mm. In the LED lamp assembly 100, however, in assembling the housing cover 112 to the heat sink 102 the housing cover 112 tends to push upward, e.g. about 0.5 mm, away from the desired height. The difficulty in achieving the desired optical tolerance in the LED lamp assembly 100 makes manufacture of the assembly 100 cumbersome and can affect optical performance if the desired height is not achieved.
Another known LED lamp assembly 600 is illustrated in FIG. 6. The illustrated assembly includes a metallic heat sink 602, an electrical connector 604, a housing cover 606 without warp, and an optic 608. In general, a PCB with LEDs and associated electronics thereon is provided between the housing cover 606 and the heat sink 602 with the LEDs on the PCB positioned for emitting light through the optic 608. Electrical energy for driving the LEDs is coupled through the connector 604 to the PCB.
As shown, the heat sink 602 includes a plurality of separate tab portions 610 around the periphery thereof. The tab portions 610 of the heat sink 602 are deformed downward against the top surface 612 of the housing cover 606, thereby compressing the housing cover 606 downward toward the heat sink 602 and the PCB disposed between the housing cover 606 and the heat sink 602. Each of the tab portions 610 imparts a separate associated compressive force on the housing cover 606 establishing a discontinuous and varying compressive force against the housing cover 606 around the periphery thereof.